An electrically heated window glass used for a windshield of vehicles has been conventionally known. The electrically heated window glass is formed by laminating two glass panels with sandwiching a heater therebetween, for example.
The electrically heated windshield may be heated by energizing the heater by means of a battery or the like, so that ice, frost, snow and the like adhered on the outside surface thereof may be melted. The inside surface of the windshield may also keep off fog thereon by heating. As a result, a filed of view may be secured in the windshield without being disturbed from ice, frost, snow and the like adhered on the outside surface, or fog caused on the inside surface thereof.
Such electrically heated window glass has been disclosed in Japanese Patent Publication No. 8-119065, for example. The electrically heated window glass disclosed in the publication comprises a transparent electrical conductive film and a pair of bus bars (i.e., electrodes for energizing) on top and bottom sides, or right and left sides thereof between two glass panels, in which the transparent electrical conductive film is energized via the bus bars by a battery or the like to heat the window glass for melting ice and snow, or defogging
However, in the case of the electrically heated windshield heated by energizing the heater, a consumptive electric power capacity of the heater must be larger than 500W in order to melt ice and the like on the outside surface of the windshield. On the other hand, such large consumptive electric power capacity required to melt ice and the like is unnecessary for defogging the inside surface of the windshield. Furthermore, the heater must be used at a lower consumptive electric power capacity for decreasing a load to the battery due to a frequent usage thereof for defogging.
When the heater energized by a battery or the like is excessively heated, an optical distortion is caused in the glass panels sandwiching the heater, and the material for constituting the heater is degraded. In order to prevent these phenomena, dedicated terminals are required for detecting an excessive heating of the glass panels sandwiching the heater.